MCF viruses are a novel group of murine C-type viruses that are invariably associated with the lymphomas that arise in high-leukemic inbred strains of mice. Biological studies by Drs. Hartley and Rowe at the National Institutes of Health have revealed that MCF isolates can be classified into two biological groups depending on the site at which the lymphoma from which they were isolated appears (after thymus primarily, or spleen and lymph nodes primarily). Using RNase T1 fingerprinting we have found that the genomes of the MCF viruses isolated by Hartley and Rowe can be classified into two groups that correspond to the two biological defined classes. By improved methods of oligonucleotide mapping two regions of the genome that distinguish the two groups of viruses have been quite precisely defined. We now plan to determine the nucleotide sequence of these regions and to construct recombinants between MCF viruses so that each region can be tested independently for its contribution to the unique biological properties of the two classes of MCF viruses.